Rear MC Sensor?

[pshaw]

Is there a way to test the sensor in the rear master cylinder on an XJ. My brake warning light is on and is annoying. I have checked the fluid level, even drained and cleaned. The only wear I noticed is the rubber, but the brakes work fine. Any suggestions? Thanks

[Jerry]

The front m/c also ties into the warning circuit. Check the level there as well. Chances are your rear cylinder is OK. In either case, a small drop in fluid level (less than an ounce) can set off the light.

If the fluid levels are OK, try unplugging the 2-wire connectors for the m/c sensors. I can't remember if the circuit is open or closed, so try jumping the bike-side pins with a paperclip or jumper wire to see if the light goes out.

[macnut xj]

If you look back about five pages, you'll find a thread titled "BRK on", where I wrote an explanation of how the system works. I did some detective work on this on my XJ fighting the same problem. If the fluid level is correct, the sensor/switch should be "made"(closed). So you can fool the system by jumpering the leads on the bike side that connect to the sensor. You can also use this method to verify which sensor is not working. (Mine was the rear.) And before you ask, AFAIK the sensors are not available separately from Yamaha.

[pshaw]

Thanks

Thanks for the reply. Well I tried the shorting out on the rear, first time with the wrong wires. I shorted the break light with which ever one is next to it. I'm thinking it was the battery sensor because now the BATT light is on. Dang it! After shorting out the correct wires the light was still on. So I tried the front ones, discovered that one wire was disconnected, the black one. The white with black stripe was connected to another white with a black stripe, seems logical. So I connected the black with the black that ties into the front turn lights. Still doesn't work, and I don't want to try shorting out more wires that for sure I don't know where they go. Well, one light on bugged me now I'm really going to be bugged. Is there a wiring diagram here or on the web? Thanks, Phill

[Jerry]

I have an XJ schematic at:
XJ Wiring - PDF

PDF format, requires Acrobat Reader. Depending on how you have your browser set, the file will either open or download to your computer. File is from Rockjoc's CD.

[macnut xj]

Well, if it's any help, the wire which connects to the battery sensor should be white with a red tracer. The two wires for the rear fluid level switch should be white with a black tracer(both on the switch and the harness side). The front sensor switch should have a white with black tracer, and a black. The harness side should be matching colors (w/blk to w/blk, and blk/blk). The black wire on the harness side should be a double female, but according to my service manual schematic, only the wire from the sensor will connect to it. The flasher black wires will connect to a double female (black) that has two wires coming out of it. However, one of those black wires goes to the double female that the sensor black connects to. Both sensors are in series with each other. Power goes from the black wire at the headlight housing, thru the front switch, back to the rear of the bike and thru the rear switch, then back to the front thru the w/blk wire to the monitor unit.
If all else fails, and you've somehow hosed the monitor control unit, let me know, as I happen to have a couple of extra instrument clusters.